Prolyl-leucyl-glycinamide (PLG) is a unique endogenous peptide that modulates specific dopamine receptor subtypes of the D2 receptor family within the CMS. The long-term goal of our research is to understand the structural basis and the molecular mechanism by which PLG modulates dopamine receptors and to use this knowledge to develop novel drugs that potentially can be used to treat Parkinson's disease, tardive dyskinesia and schizophrenia. In pursuing this objective, the location of the PLG allosteric binding site on the dopamine D2L receptor will be determined and the structural nature of this allosteric binding site delineated. The general approach to accomplish this specific aim will be to use a family of bifunctional photoaffinity labeling agents wtth affinity for the allosteric PLG modulatory site. These photoaffinity labeling agents will be cross-linked with the dopamine D2L receptor and the cross-linked biotinylated fragments will be isolated and their sequences determined by mass spectrometry to identify the residues modified with the cross-linker. In a second specific aim, a library of 170 PLG analogues and peptidomimetics will be evaluated in a PLG receptor binding assay. The data from this assay will be used to construct pharmacophore and three-dimensional quantitative structure-activity relationship models of the PLG binding site. These models will be used with the data obtained in the first specific aim on the structural nature of the PLG binding site to develop a model of the PLG allosteric binding site from which hypotheses can be formulated to determine the mechanism of dopamine receptor modulation by PLG. The nature of the 28 KDa protein that also is labeled by the PLG photoaffinity labeling agents will be identified and characterized. In specific aim four, the structural features behind the negative modulatory activity of a spiro bicyclic peptidomimetic will be determined. Analogues will be made to test the hypothesis that the negative modulatory activity of the spiro bicyclic peptidomimetic is brought about by the beta-methylene group of the thiazolidine ring of this molecule projecting into a different area of topological space, thereby inducing a different conformational change than that induced with the positive modulators. Finally, the effects of PLG peptidomimetics on the reversal and prevention of behavioral abnormalities in preclinical animal models of schizophrenia will be investigated as part of our long-term goal to explore the potential of PLG and its peptidomimetics in treating diseases in which dopamine receptors are known to play a role. In addition to the possible therapeutic importance of this area of research, the modulation of dopamine receptors by PLG represents a novel means of regulating receptor activity that has counterparts in other G-protein coupled receptor systems. Thus, the research being conducted on the interaction between PLG and dopamine receptors will provide further molecular insights into G-protein receptor mechanisms and possible ways of regulating this very important superfamily of receptors.